Matrix addressable, flat panel displays are widely used in a variety of applications, including computer displays. One type of device well suited for such applications is the field emission display. Field emission displays typically include a generally planar baseplate beneath a faceplate. The baseplate includes a substrate having an array of projecting emitters. Usually, the emitters are conical projections integral to the substrate and may be grouped into emitter sets where the bases of emitters are commonly connected.
A conductive extraction grid is positioned above the emitters are driven with a voltage of about 30-120 V. The emitters are then selectively activated by providing electrons to the emitters, thereby allowing electrons to be drawn from the emitters by the extraction grid voltage. If the voltage differential between the emitters and the extraction grid is sufficiently high, the resulting electric field extracts electrons from the emitters.
The faceplate is mounted directly adjacent the extraction grid and includes a transparent display screen coated with a transparent conductive material to form an anode that is biased to about 1-2 kV. A cathodoluminescent layer covers the exposed surface of the anode. Electrons emitted by the emitters are attracted by the anode and strike the cathodoluminescent layer, causing the cathodoluminescent layer to emit light at the impact site. The emitted light then passes through the anode and the glass plate where it is visible to a viewer. The brightness of the light produced in response to the emitted electrons depends, in part, upon the number of electrons striking the cathodoluminescent layer in an activation interval, which in turn depends upon the current flow to the emitters. The brightness of each area can thus be controlled by controlling the current flow to the respective emitter or emitter set. The light from each area of the display can thus be controlled to produce an image. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel."
Typically, current flow to the emitters is controlled by controlling the voltage applied to the bases of the emitters to produce a selected voltage differential between the emitters and the extraction grid to produce an intense electric field. The magnitude of the current to the emitters then corresponds to the intensity of the electric field as determined by the voltage differential.
One problem with the above-described approach is that the response of the emitter sets to applied grid and emitter voltages may be non-uniform. Typically, this is caused by variations in the separations between the emitters and the extraction grid across the array, which causes differences in the electric field intensity for a given voltage difference. Often, these separation variations result from variations in the diameter of apertures into which the emitters project, which in turn, are caused by processing variations. Consequently, for a given voltage differential between the emitters and the extraction grid, the brightness of the emitted light may vary according to the location of the emitters.
One way to address such variations may be to employ relatively complex circuitry to fixedly set current through each of the emitters. However, the number of emitters in a field emission display can be substantial. Consequently, simplification of the circuitry for each of the emitters can produce a substantial benefit in overall cost and complexity of the display.